1. Field of the Invention
This invention relates to a phase difference compensating device for eliminating the problems from occurring in that, in cases where an image displayed on an image display device utilizing a twisted nematic liquid crystal (hereinbelow referred to as the TN liquid crystal) is seen, image contrast becomes low in accordance with an field of view. This invention also relates to a liquid crystal apparatus, such as a liquid crystal projector, using the phase difference compensating device.
2. Description of the Related Art
Liquid crystal display devices operating in various different operation modes have heretofore been known. In particular, for the advantages with regard to mass production, the TN liquid crystal display devices have become popular as image display devices of direct viewing types of flat panel display apparatuses and liquid crystal projectors. The TN liquid crystal display devices comprise a pair of transparent base plates, on each of which a transparent electrode and an orientating film have been formed, and rod-shaped liquid crystal molecules, which constitute a liquid crystal layer and have been filled between the pair of the transparent base plates. The liquid crystal molecules are kept such that the major axes of the liquid crystal molecules are parallel with the base plates, such that the directions of the major axes of the liquid crystal molecules are tilted little by little over the thickness direction of the liquid crystal layer, and such that the liquid crystal molecules are set in a state of an orientation twisted by an angle of 90° as a whole.
In the state in which the liquid crystal molecules are thus orientated in the liquid crystal layer, linearly polarized light may be caused to impinge upon the liquid crystal layer from the side of one of the base plates. In such cases, during the travel of the linearly polarized light through the liquid crystal layer toward the other base plate, the direction of polarization of the linearly polarized light is rotated by an angle of 90° in accordance with the state of orientation of the liquid crystal molecules. The linearly polarized light, whose direction of polarization has thus been rotated by an angle of 90°, is radiated out from the liquid crystal layer.
Also, in cases where a voltage is applied across the liquid crystal layer, the aforesaid twisted orientation of the liquid crystal molecules is lost, and the liquid crystal molecules are set in a state of orientation, in which the major axes of the majority of the liquid crystal molecules are erected in the direction normal to the base plate. In this state, the linearly polarized light may be caused to impinge upon the liquid crystal layer from the side of one of the base plates. In such cases, the linearly polarized light is radiated out from the liquid crystal layer in the state, in which the direction of polarization of the linearly polarized light is not altered.
A pair of polarizing plates may be located on the light incidence side and the light radiating side of the TN liquid crystal display device, such that the directions of polarization may be normal to each other. (The aforesaid location of the polarizing plates is referred to as the crossed Nicols arrangement.) In such cases, in the state in which the voltage is not applied across the liquid crystal layer, the direction of polarization of the linearly polarized light, which has passed through the polarizing plate located on the light incidence side of the TN liquid crystal display device and has impinged upon the liquid crystal layer, is rotated by an angle of 90° by the effect of the liquid crystal molecules. Therefore, in such cases, the linearly polarized light is radiated out from the polarizing plate located on the light radiating side of the TN liquid crystal display device, and the TN liquid crystal display device is thus set in a bright state. (The aforesaid operation mode of the TN liquid crystal display device is referred to as the normally white mode.) Also, in the state in which the voltage is applied across the liquid crystal layer, the direction of polarization of the linearly polarized light, which has passed through the polarizing plate located on the light incidence side of the TN liquid crystal display device and has impinged upon the liquid crystal layer, is kept without being altered. Therefore, in such cases, the linearly polarized light is blocked by the polarizing plate located on the light radiating side of the TN liquid crystal display device, and the TN liquid crystal display device is thus set in a dark state.
Also, a pair of polarizing plates may be located on the light incidence side and the light radiating side of the TN liquid crystal display device, such that the directions of polarization may be parallel with each other. (The aforesaid location of the polarizing plates is referred to as the parallel Nicols arrangement.) In such cases, in the state in which the voltage is not applied across the liquid crystal layer, the TN liquid crystal display device is set in the dark state. (The aforesaid operation mode of the TN liquid crystal display device is referred to as the normally black mode.) Further, in the state in which the voltage is applied across the liquid crystal layer, the TN liquid crystal display device is set in the bright state. The TN liquid crystal display device is also capable of being operated in the normally black mode.
Ordinarily, the TN liquid crystal display device has the drawbacks in that the field of view is narrow. The aforesaid drawbacks of the TN liquid crystal display device arise since the liquid crystal molecules act also as birefringent media. By way of example, in the cases of the TN liquid crystal display device of the normally white mode, at the stage at which the voltage is applied across the liquid crystal layer, and at which the twisted orientation of the liquid crystal molecules is thereby lost, both the optical rotatory power and the birefringent characteristics prevail. As the applied voltage becomes high, the birefringent characteristics become dominant. Also, at the time at which the twisted orientation of the liquid crystal molecules has been lost, and at which the TN liquid crystal display device is thus set in the dark state, the liquid crystal layer exhibits little birefringent characteristics with respect to normal incident light, and therefore the linearly polarized light, which impinges upon the liquid crystal layer from the normal direction, directly passes through the liquid crystal layer. However, at the time at which the twisted orientation of the liquid crystal molecules has been lost, and at which the TN liquid crystal display device is thus set in the dark state, the liquid crystal layer exhibits the birefringent characteristics with respect to oblique incident light, and therefore the linearly polarized light, which impinges upon the liquid crystal layer from the oblique direction, is modulated into elliptically polarized light. Part of the elliptically polarized light, which has thus been produced, passes through the polarizing plate, which is located on the light radiating side of the TN liquid crystal display device. As a result, the extent of the dark state of the TN liquid crystal display device becomes low. The characteristics of the liquid crystal layer acting as the birefringent medium occur little by little also at the stage of transfer from the bright state to the dark state of the TN liquid crystal display device. Therefore, in the state in which a continuous tone image is displayed, in cases where the display screen is seen from an oblique direction, dependence of a modulation degree upon the angle is not capable of being avoided.
In cases where the TN liquid crystal display device is employed as the image display device of the direct viewing type of the flat panel display apparatus, the aforesaid field angle characteristics of the TN liquid crystal display device cause a phenomenon to occur in that a black image density and a tint vary for different directions of seeing. Also, in cases where the TN liquid crystal display device is employed as the image display device of the liquid crystal projector, the aforesaid field angle characteristics of the TN liquid crystal display device cause a phenomenon to occur in that the contrast of the image projected onto a screen becomes low. In both the cases described above, the aforesaid field angle characteristics of the TN liquid crystal display device cause the image quality of the displayed image to become markedly low.
The aforesaid drawbacks of the TN liquid crystal display device are capable of being suppressed by the utilization of a multi-layer thin film comprising a thin film constituted of a high refractive index material and a thin film constituted of a low refractive index material, which thin films are laminated alternately, such that an optical film thickness falls within the range of one-hundredth of light wavelengths to one-fifth of light wavelengths. (The aforesaid multi-layer thin film is described in, for example, Japanese Unexamined Patent Publication No. 2004-102200.) The aforesaid multi-layer thin film has negative C-plate characteristics. In cases where the linearly polarized light impinges from an oblique direction upon the liquid crystal layer, in which the liquid crystal molecules take a normal orientated attitude for dark state displaying, and the linearly polarized light is thus subjected to the birefringence, the aforesaid multi-layer thin film acts as a negative uniaxial birefringent material, which compensates for a phase difference between ordinary rays and extraordinary rays in accordance with the angle of incidence of the linearly polarized light. Therefore, with the multi-layer thin film described above, the elliptically polarized light having been produced due to the birefringence is returned to the linearly polarized light, and the problems are capable of being prevented from occurring in that leak light is radiated out from the post-stage polarizing plate. Also, the aforesaid multi-layer thin film acting as the phase difference compensating device has the feature in that the multi-layer thin film is capable of being constituted of inorganic materials. The aforesaid multi-layer thin film has a high heat resistance, a high light resistance, a high physical stability, and a high chemical stability. Accordingly, the aforesaid multi-layer thin film is capable of being utilized efficiently for the direct viewing types of the flat panel display apparatuses and the liquid crystal projectors.
Further, it has been known that an O-plate is efficient for improvement of the field angle characteristics of the TN liquid crystal display device. (The efficiency of the O-plate is described in, for example, U.S. Pat. No. 5,638,197.) The O-plate is a birefringent material, in which the main optic axis, which does not cause the birefringence to occur, is tilted with respect to a reference surface (e.g., the base plate surface of the liquid crystal display device). In U.S. Pat. No. 5,638,197, it is also disclosed that the O-plate is capable of being produced easily with an oblique incidence vacuum evaporation technique, in which an inorganic material is vacuum deposited from an oblique direction with respect to the base plate, and that the O-plate may be utilized in combination with the C-plate or an A-plate.
Furthermore, each of WV film (trade name) and WV-SA film (trade name) has been used in practice. (Each of the WV film and the WV-SA film is described in, for example, “Development of Wide View SA, a Film Product Widening the Viewing Angle of LCDs” by Hiroyuki Mori, et al., FUJIFILM RESEARCH & DEVELOPMENT, No. 46-2001, pp. 51-55.) Each of the WV film and the WV-SA film comprises a TAC film, which acts as a base, and a disk-shaped liquid crystal compound, which is fixed in a state of hybrid orientation to the TAC film. With each of the WV film and the WV-SA film, in cases where the dark state displaying is performed, the majority of the liquid crystal molecules, which are distributed in the thickness direction of the liquid crystal layer, take the normal orientated attitude, and the liquid crystal molecules, which are located in a region close to the base plate, undergo a hybrid orientation such that the major axes of the liquid crystal molecules are erected little by little from the orientated attitude, which is approximately parallel with the base plate. With the birefringent material described in Japanese Unexamined Patent Publication No. 2004-102200, the phase difference compensation is not perfect with respect to the birefringence due to the liquid crystal molecules, which are located in the region close to the base plate. However, with each of the WV film and the WV-SA film, wherein the disk-shaped liquid crystal compound undergoes the hybrid orientation, efficient phase difference compensation is capable of being performed also with respect to the birefringence due to the liquid crystal molecules, which are located in the region close to the base plate.
As one of techniques for producing a phase difference compensating device constituted of an inorganic material, there has been proposed a technique, wherein an oblique incidence vacuum deposited film is formed with oblique incidence vacuum evaporation of the inorganic material onto a base plate, and wherein the thus formed oblique incidence vacuum deposited film is utilized as a phase difference compensating layer. (The aforesaid technique for producing a phase difference compensating device constituted of an inorganic material is disclosed in, for example, each of Japanese Unexamined Patent Publication No. 8 (1996)-122523 and U.S. Pat. No. 5,932,354.)
As described above, with the phase difference compensating device described in Japanese Unexamined Patent Publication No. 2004-102200, in cases where the dark state displaying is performed by use of the TN liquid crystal display device in the normally white mode, as for the majority of the liquid crystal molecules, which are orientated in the normal direction within the liquid crystal layer, the phase difference compensation is made appropriately with respect to the light beam incident from the oblique direction. However, with the phase difference compensating device described in Japanese Unexamined Patent Publication No. 2004-102200, the problems are encountered in that, in such cases, as for the liquid crystal molecules, which undergo the hybrid orientation in the region close to the base plate, the phase difference compensation is not perfect, and in that an improvement need be made even further.
The O-plate described in U.S. Pat. No. 5,638,197 is constituted of the single-layer, oblique incidence vacuum deposited film. However, in order for the O-plate to be used in practice, a study need be made even further with regard to how the structure of the oblique incidence vacuum deposited film is to be optimized for obtaining the desired field angle characteristics in cases where the O-plate is utilized alone or in combination with the C-plate, or the like.
Each of the WV film and the WV-SA film described in “Development of Wide View SA, a Film Product Widening the Viewing Angle of LCDs” by Hiroyuki Mori, et al., FUJIFILM RESEARCH & DEVELOPMENT, No. 46-2001, pp. 51-55 is capable of performing efficient phase difference compensation. However, each of the WV film and the WV-SA film is principally constituted of an organic material, and therefore has the problems with regard to the durability. In cases where each of the WV film and the WV-SA film is exposed for a long period of time to strong light containing ultraviolet light, each of the WV film and the WV-SA film is apt to suffer from color fading. Particularly, in cases where each of the WV film and the WV-SA film is utilized for the liquid crystal projectors, since the luminance of the light source is set to be high for the image projection onto the screen, and since the extent of heating becomes high, the problems occur in that browning occurs little by little during a use period of approximately 2,000 hours to 3,000 hours.
In order for the aforesaid problems to be eliminated, it will be desired that the phase difference compensating device, which is described in Japanese Unexamined Patent Publication No. 2004-102200, be imparted with the hybrid orientation. However, the phase difference compensating device made from the inorganic material undergoing the hybrid orientation is not always capable of being produced easily and being used in practice. It will also be desired that the negative C-plate, which is described in Japanese Unexamined Patent Publication No. 2004-102200, and the O-plate, which is described in U.S. Pat. No. 5,638,197, be utilized in combination. However, currently, nothing has been reported with regard to information concerning a definite constitution for the combination of the C-plate and the O-plate and a practical effect of the combination of the C-plate and the O-plate, and a product utilizing the combination of the C-plate and the O-plate has not yet been produced.
As for the phase difference compensating device made from the inorganic material as disclosed in each of Japanese Unexamined Patent Publication No. 8(1996)-122523 and U.S. Pat. No. 5,932,354, nothing has heretofore been reported with regard to a practical effect obtained from the combination of the phase difference compensating device with the TN liquid crystal display device, and therefore it will not always be possible to utilize the phase difference compensating device for the phase difference compensation of the TN liquid crystal display device. Also, the technique for laminating the oblique incidence vacuum deposited film is disclosed in each of Japanese Unexamined Patent Publication No. 8(1996)-122523 and U.S. Pat. No. 5,932,354. However, with the disclosed technique for laminating the oblique incidence vacuum deposited film, the problems are encountered in that the surface characteristics of the phase difference compensating device become bad, and in that optical performance as designed is not capable of being obtained. Therefore, the disclosed technique for laminating the oblique incidence vacuum deposited film is not always practicable.